Multiblog + unique communities

Being a new user I have been playing around with Buddypress and managed to set up a Multiblog install which runs a number of subdomain installs, all using the default Buddypress-enabled theme (for now), as in:
site1.mydomain.com
site2.mydomain.com
site3.mydomain.com
(hint to new users: if you want a buddypress theme to work on all these subdomains, don’t forget to enable this line in wp-config:
define( ‘BP_ENABLE_MULTIBLOG’, true); It took me 2 days to figure that out)

Now, I am happy with the possibility of running different sites while sharing a community, however I’d like to differentiate between the users that signed up through these different sub-sites. For example, a user on site2 should be able to fully interact with a user who signed up on site1, BUT I’d like to have a way to list only the users that became members through either site 1, 2 or 3.
In that way, site1 would have a unique set of users that signed up through the site1 subdomain, and the same for the other subdomains.

If I understand correctly, there is currently no way to distinguish between users that signed up through these different subdomain sites?
Are there any plugins out there, or has anyone worked on making this possible? ‘Flagging’ a user so to say, so that they can always be filtered as being part of the ‘site1’ group of users. For the subsite admins this would offer the advantage of being able to choose between showing only members of their own site, or the members of the overall system (site1, 2 and 3 combined).

To my knowledge no such plugin exists but it makes since. I thought I read something a couple of weeks ago during the bi-weekly BP dev chat that someone was working on something like that. I believe they called it BP Multi-user (i.e. BPMU).

Wow, I could really use something like that at my geotagging photoblog at FavTrip.com!

Even without being able to track users coming in through the subdomains – yes, that’d be nice (maybe not through Buddypress but through analytics or something? I don’t know) – I really need to set up multiple users with my site. I just got featured on KillerStartups.com and am trying to get all the functionality worked out!

Just so you know I’m in the process of setting up my first WP/BP site so I’m a complete beginner and this might be bovine fecal matter.

Not sure this is what you want (or if it will even work), but it might be a workaround to get started.

You can set users to autojoin a specific bpgroup on registration, name the groups site1, site2, etc and when they signup through each sub they can be automatically joined to that group. (and automatically ‘friend’ the members of that group if you want)

That way you can differentiate the members of each site by which group they joined. Like I said, this might be bovine…… but it might work too

Thanks everyone. BPMU sounds interesting, though I haven’t found anything about it online so far. The multi-network plugin is very cool as well, especially the idea of having several super admins, will keep an eye on that.
Pagan Gwynne: the groups solution is pretty smart, for now that’s a first step to what I want to achieve so I will test it out and post my experiences here.

This plugin does the trick. The only issue is that it hasn’t been updated to work with Buddypress so you need to manually change the ‘wpmu-admin’ references into ‘ms-admin’ by simply editing the script.